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Castle.Core.AsyncInterceptor

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Library to simplify interception of asynchronous methods

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# Castle.Core.AsyncInterceptor

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## What is AsyncInterceptor?

__AsyncInterceptor__ is an extension to [Castle DynamicProxy](http://www.castleproject.org/projects/dynamicproxy/) to
simplify the development of interceptors for asynchronous methods.

## Why do I want intercept methods?

The whys and wherefores of implementing interceptors is lengthy discussion, and beyond the scope of this introduction.
A very common scenario is in the implementation of
[Aspect-oriented patterns](https://en.wikipedia.org/wiki/Aspect-oriented_programming), for which exception handling is
useful use case.

An interceptor that catches exceptions and logs them could be implemented quite simply as:

```csharp
// Intercept() is the single method of IInterceptor.
public void Intercept(IInvocation invocation)
{
    try
    {
        invocation.Proceed();
    }
    catch (Exception ex)
    {
        Log.Error($"Error calling {invocation.Method.Name}.", ex);
        throw;
    }
}
```

## What's not simple about asynchronous method interception?

When implementing `IInterceptor` the underlying method is invoked like this:

```csharp
public void Intercept(IInvocation invocation)
{
    // Step 1. Do something prior to invocation.

invocation.Proceed();

// Step 2. Do something after invocation.
}
```

For synchronous methods `Proceed()` returns only when the underlying method completes, but for asynchronous methods,
(those that return [`Task`](https://msdn.microsoft.com/en-us/library/system.threading.tasks.task.aspx) or
[`Task<TResult>`](https://msdn.microsoft.com/en-us/library/dd321424.aspx))
the `Proceed()` returns as soon as the underlying method hits an `await` (or `ContinueWith`).

Therefore with asynchronous methods _Step 2_ is executed before the underlying methods logically completes.

## How to intercept asynchronous methods without AsyncInterceptor?

To demonstrate how __AsyncInterceptor__ simplifies the interception of asynchronous methods, let's show how to do it
without __AsyncInterceptor__.

### Methods that return [`Task`](https://msdn.microsoft.com/en-us/library/system.threading.tasks.task.aspx)

To intercept methods that return a [`Task`](https://msdn.microsoft.com/en-us/library/system.threading.tasks.task.aspx)
(__Note:__ it must be a [`Task`](https://msdn.microsoft.com/en-us/library/system.threading.tasks.task.aspx) not
[`Task<TResult>`](https://msdn.microsoft.com/en-us/library/dd321424.aspx)) is not overly complicated.

The invocation provides access to the return value. By checking the type of the return value it is possible to await
the completion of the [`Task`](https://msdn.microsoft.com/en-us/library/system.threading.tasks.task.aspx).

```csharp
public void Intercept(IInvocation invocation)
{
    // Step 1. Do something prior to invocation.

invocation.Proceed();
    Type type = invocation.ReturnValue?.GetType();
    if (type != null && type == typeof(Task))
    {
        // Given the method returns a Task, wait for it to complete before performing Step 2
        Func<Task> continuation = async () =>
        {
            await (Task) invocation.ReturnValue;

// Step 2. Do something after invocation.
        };

invocation.ReturnValue = continuation();
        return;
    }

// Assume the method is synchronous.

// Step 2. Do something after invocation.
}
```

### Methods that return [`Task<TResult>`](https://msdn.microsoft.com/en-us/library/dd321424.aspx)

To intercept methods that return a [`Task<TResult>`](https://msdn.microsoft.com/en-us/library/dd321424.aspx) is far
from simple. It's the reason why I created this library. Rather than go into the detail (the solution requires the use
of reflection) the stack overflow question [Intercept async method that returns generic Task<> via
DynamicProxy](https://stackoverflow.com/a/43272955) provides great overview.

## How to intercept asynchronous methods __with__ AsyncInterceptor?

If you've got this far, then it's probably safe to assume you want to intercept asynchronous methods, and the options
for doing it manually look like a lot of work.

### Option 1:  Implement `IAsyncInterceptor` interface to intercept invocations

Create a class that implements `IAsyncInterceptor`, then register it for interception in the same way as `IInterceptor`
using the ProxyGenerator extension methods, e.g.

```csharp
var myClass = new ClasThatImplementsIMyInterface();
var generator = new ProxyGenerator();
var interceptor = new ClasThatImplementsIAsyncInterceptor();
IMyInterface proxy = generator.CreateInterfaceProxyWithTargetInterface<IMyInterface>(myClass, interceptor)
```

Implementing
[IAsyncInterceptor](https://github.com/JSkimming/Castle.Core.AsyncInterceptor/blob/master/src/Castle.Core.AsyncInterceptor/IAsyncInterceptor.cs)
is the closest to traditional interception when implementing
[IInterceptor](https://github.com/castleproject/Core/blob/master/src/Castle.Core/DynamicProxy/IInterceptor.cs)

Instead of a single `void Intercept(IInvocation invocation)` method to implement, there are three:

```csharp
void InterceptSynchronous(IInvocation invocation);
void InterceptAsynchronous(IInvocation invocation);
void InterceptAsynchronous<TResult>(IInvocation invocation);
```

#### `InterceptSynchronous(IInvocation invocation)`

`InterceptSynchronous` is effectively the same as `IInterceptor.Intercept`, though it is only called for synchronous
methods, e.g. methods that do not return `Task` or `Task<TResult>`.

Implementing `InterceptSynchronous` could look something like this:

```csharp
public void InterceptSynchronous(IInvocation invocation)
{
    // Step 1. Do something prior to invocation.

invocation.Proceed();

// Step 2. Do something after invocation.
}
```

#### `InterceptAsynchronous(IInvocation invocation)`

`InterceptAsynchronous(IInvocation invocation)` is called for methods that return `Task` but not the generic
`Task<TResult>`.

Implementing `InterceptAsynchronous(IInvocation invocation)` could look something like this:

```csharp
public void InterceptAsynchronous(IInvocation invocation)
{
    invocation.ReturnValue = InternalInterceptAsynchronous(invocation);
}

private async Task InternalInterceptAsynchronous(IInvocation invocation)
{
    // Step 1. Do something prior to invocation.

invocation.Proceed();
    var task = (Task)invocation.ReturnValue;
    await task;

// Step 2. Do something after invocation.
}
```

#### `InterceptAsynchronous<TResult>(IInvocation invocation)`

`InterceptAsynchronous<TResult>(IInvocation invocation)` is called for methods that return the generic `Task<TResult>`.

Implementing `InterceptAsynchronous<TResult>(IInvocation invocation)` could look something like this:

```csharp
public void InterceptAsynchronous<TResult>(IInvocation invocation)
{
    invocation.ReturnValue = InternalInterceptAsynchronous<TResult>(invocation);
}

private async Task<TResult> InternalInterceptAsynchronous<TResult>(IInvocation invocation)
{
    // Step 1. Do something prior to invocation.

invocation.Proceed();
    var task = (Task<TResult>)invocation.ReturnValue;
    TResult result = await task;

// Step 2. Do something after invocation.

return result;
}
```

### Option 2: Extend `AsyncInterceptorBase` class to intercept invocations

#### __:warning: PROCEED WITH CAUTION :warning: This option come with a major caveat.__

> If you need to perform asynchronous operations before calling proceed, then you should implement option 1, otherwise
> you risk  thread starvation and deadlocking.

Create a class that extends the abstract base class `AsyncInterceptorBase`, then register it for interception in the same way as `IInterceptor` using the ProxyGenerator extension methods, e.g.

```csharp
var myClass = new ClasThatImplementsIMyInterface();
var generator = new ProxyGenerator();
var interceptor = new ClasThatExtendsAsyncInterceptorBase();
IMyInterface proxy = generator.CreateInterfaceProxyWithTargetInterface<IMyInterface>(myClass, interceptor)
```

Extending `AsyncInterceptorBase` provides a simple mechanism to intercept methods using the __async/await__ pattern. There are two abstract methods that must be implemented.

```csharp
Task InterceptAsync(IInvocation invocation, Func<IInvocation, Task> proceed);
Task<T> InterceptAsync<T>(IInvocation invocation, Func<IInvocation, Task<T>> proceed);
```

Each method takes two parameters. The `IInvocation` provided by __DaynamicProxy__ and a proceed function to execute the invocation returning an awaitable task.

The first method in called when intercepting `void` methods or methods that return `Task`. The second method is called when intercepting any method that returns a value, including `Task<TResult>`.

A possible extension of `AsyncInterceptorBase` for exception handling could be implemented as follows:

```csharp
public class ExceptionHandlingInterceptor : AsyncInterceptorBase
{
    protected override async Task InterceptAsync(IInvocation invocation, Func<IInvocation, Task> proceed)
    {
        try
        {
            // Cannot simply return the the task, as any exceptions would not be caught below.
            await proceed(invocation).ConfigureAwait(false);
        }
        catch (Exception ex)
        {
            Log.Error($"Error calling {invocation.Method.Name}.", ex);
            throw;
        }
    }

protected override async Task<T> InterceptAsync<T>(IInvocation invocation, Func<IInvocation, Task<T>> proceed)
    {
        try
        {
            // Cannot simply return the the task, as any exceptions would not be caught below.
            return await proceed(invocation).ConfigureAwait(false);
        }
        catch (Exception ex)
        {
            Log.Error($"Error calling {invocation.Method.Name}.", ex);
            throw;
        }
    }
}
```

### Option 3: Extend `ProcessingAsyncInterceptor<TState>` class to intercept invocations

Create a class that extends the abstract base class `ProcessingAsyncInterceptor<TState>`, then register it for
interception in the same was as `IInterceptor` using the ProxyGenerator extension methods, e.g.

```csharp
var myClass = new ClasThatImplementsIMyInterface();
var generator = new ProxyGenerator();
var interceptor = new ClasThatExtendsProcessingAsyncInterceptor();
IMyInterface proxy = generator.CreateInterfaceProxyWithTargetInterface<IMyInterface>(myClass, interceptor)
```

Extending
[`ProcessingAsyncInterceptor<TState>`](https://github.com/JSkimming/Castle.Core.AsyncInterceptor/blob/documentation/src/Castle.Core.AsyncInterceptor/ProcessingAsyncInterceptor.cs),
provides a simplified mechanism of intercepting method invocations without having to implement the three methods of
`IAsyncInterceptor`.

`ProcessingAsyncInterceptor<TState>` defines two virtual methods, one that is invoked before to the method invocation,
the second after.

```csharp
protected virtual TState StartingInvocation(IInvocation invocation);
protected virtual void CompletedInvocation(IInvocation invocation, TState state);
```

State can be maintained between the two method through the generic class parameter `TState`. `StartingInvocation` is
called before method invocation. The return value of type `TState` is then passed to the `CompletedInvocation` which is
called after method invocation.

A possible extension of `ProcessingAsyncInterceptor<TState>` could be as follows:

```csharp
public class MyProcessingAsyncInterceptor : ProcessingAsyncInterceptor<string>
{
    protected override string StartingInvocation(IInvocation invocation)
    {
        return $"{invocation.Method.Name}:StartingInvocation:{DateTime.UtcNow:O}";
    }

protected override void CompletedInvocation(IInvocation invocation, string state)
    {
        Trace.WriteLine(state);
        Trace.WriteLine($"{invocation.Method.Name}:CompletedInvocation:{DateTime.UtcNow:O}");
    }
}
```

The state of type `TState` returned from `StartingInvocation` can be `null`. Neither `StartingInvocation` nor
`CompletedInvocation` are require to be overridden in the class that derives from `ProcessingAsyncInterceptor<TState>`.
The default implementation of StartingInvocation simply returns `null`. If all you require is to intercept methods
after they are invoked, then just implement `CompletedInvocation` and ignore the `state` parameter which will be
null. In that situation your class can be defined as:

```csharp
public class MyProcessingAsyncInterceptor : ProcessingAsyncInterceptor<object>
{
    protected override void CompletedInvocation(IInvocation invocation, object state)
    {
        Trace.WriteLine($"{invocation.Method.Name}:CompletedInvocation:{DateTime.UtcNow:O}");
    }
}
```

## Using AsyncInterceptor with non-overloaded `ProxyGenerator` methods

While AsyncInterceptor offers convenient overloads for the most common `ProxyGenerator` methods, some methods do not (yet) have such overloads. In this case, the extension method `IAsyncInterceptor.ToInterceptor()` can be used to obtain a regular `IInterceptor` implementation.

```csharp
var generator = new ProxyGenerator();
var interceptor = new MyInterceptorWithoutTarget<T>();
generator.CreateInterfaceProxyWithoutTarget(typeof(T), interceptor.ToInterceptor());
```

## Method invocation timing using `AsyncTimingInterceptor`

A common use-case for method invocation interception is to time how long a method takes to execute. For this reason
[`AsyncTimingInterceptor`](https://github.com/JSkimming/Castle.Core.AsyncInterceptor/blob/documentation/src/Castle.Core.AsyncInterceptor/AsyncTimingInterceptor.cs)
is provided.

`AsyncTimingInterceptor` is a specialised implementation of `ProcessingAsyncInterceptor<TState>` that uses a
[Stopwatch](https://msdn.microsoft.com/en-us/library/system.diagnostics.stopwatch.aspx) as the `TState`.

`AsyncTimingInterceptor` defines two abstract methods, one that is invoked before method invocation and before the
Stopwatch has started. The second after method invocation and the Stopwatch has stopped

```csharp
protected abstract void StartingTiming(IInvocation invocation);
protected abstract void CompletedTiming(IInvocation invocation, Stopwatch stopwatch);
```

A possible extension of `AsyncTimingInterceptor` could be as follows:

```csharp
public class TestAsyncTimingInterceptor : AsyncTimingInterceptor
{
    protected override void StartingTiming(IInvocation invocation)
    {
        Trace.WriteLine($"{invocation.Method.Name}:StartingTiming");
    }

protected override void CompletedTiming(IInvocation invocation, Stopwatch stopwatch)
    {
        Trace.WriteLine($"{invocation.Method.Name}:CompletedTiming:{stopwatch.Elapsed:g}");
    }
}
```

## Testing

This library maintains a high level of code coverage with extensive unit tests.

### Running the tests

There are several ways to run the tests, the most convenient is through Visual Studio, via Test Explorer or ReSharper.

The tests can also be executed from the command line like this:

```bash
dotnet test test/Castle.Core.AsyncInterceptor.Tests/Castle.Core.AsyncInterceptor.Tests.csproj
```

On Windows the above command will execute the tests on all 3 runtimes, .NETFramework,Version=v4.7,
.NETCoreApp,Version=v1.1, and .NETCoreApp,Version=v2.1.

To run the tests targeting a specific runtime (which may be necessary if you don't have all them all installed) run the
following command:

```bash
dotnet test -f netcoreapp3.1 test/Castle.Core.AsyncInterceptor.Tests/Castle.Core.AsyncInterceptor.Tests.csproj
```

A docker compose file is provided to run the tests on a linux container. To execute the tests in a container run the
following command:

```bash
docker build --target test --progress=plain .
```

### Executing the code coverage tests

Code coverage uses the excellent and free [OpenCover](https://github.com/OpenCover/opencover "OpenCover Repository").

To execute the tests with code coverage (Windows Only) run the following command:

```cmd
coverage.cmd
```

Code coverage reports are produced using
[ReportGenerator](https://github.com/danielpalme/ReportGenerator "ReportGenerator Repository") and can be viewed in the
`test/TestResults/Report` folder.